Process of producing white pulp of high strength



Patented July 12, 1927.

UNITED" STATES PATENT OFFICE.

GEORGE A. RICHTER, F BERLIN, NEW HAMPSHIRE, ASSIGNOR 'ICl BROWN COMPANY,

OF BERLIN, NEW HAMPSHIRE, A CORPORATION OF MAINE.

IPI'\-OCEiSS 0F PRODUCING WHITE PULP OF HIGH STRENGTH."

No Drawing.

U. S. Letters Patent Serial No. 749,564, filed November 12, 1924. In that application, the salient feature of discovery, generally stated, is that if wood chips (spruce, for

/ example), are cooked in an acid sulphite solution in which the free S0 and the combined So -are in substantially equal proportions (or the free SO less in proportion than the combined S0 but are present in sufficient quantity in solution to act upon the encrusting or cementitious content ,of the wood for a sufficient period of time in a closed digester at a temperature higher than that ordinarily employed in sulphite digestion, a pulp is producedwhich has marked characteristics and possesses certain features of advantage over the usual or commercial form of sulphite. pulp. Such pulp is especially characterized by its light color, ex-

traordinary strength, length of fiber, and its high'lignin and pentosan content, and, for convenience of designation, will be termed strong sulphite, pulp, in contradistinction to the ordinary sulphite pul p. Its strength is probably due'to its high ligneous and pentosan content, which approximates that of kraft or sulphate pulp. The pentosans, probably due to their adhesive or sticky qualities, act as a binder tosbind the constituent fibers strongly together, thereby imparting strength to the ulp. However, owing: to such high .lignln content, the strong pulp is very difiicult to bleach, re-

'quiring about 25% to 35% of 35% bleach, (based on the bone-dry weight of pulp), to

produce a bleached product which has the color of ordinary bleached sulphite pulp, but, owing to the use of excess bleach, the strength of the original pulp is materially reduced,-prob*ably due to the degradation of a portion of the cellulose into oxycellulose, so that ordinarily it would not be economical content.

Application filed July 11,1925. Serial No. 42,977.

to bleach strong sulphite pulp for the production ofa white pulp incompetition with ordinary bleached sulphite pulp.

The primary object of the present invention is to produce a white pulp of high strength from strong sulphite pulp. To attain this object, the present invention must provide for the removal of the ligneous or .-colori11g substance in the pulp without materially affecting its fiber strength or pentosan I have discovered that if strong sulphite pulp is treated with an oxidizing solution containing oxidizing agent in such quantity that a removal of lignin is effected without causing a substantial change in the strength of the pulp or its pentosan'content, and is subsequently digested at an elevated temperature in an alkaline solution containing alkaline agent in such quantity as to effect a further removal of impurities in the pulp without affecting its strength, a prod- .uct results which may be whitened with the usual moderate amount of bleach to give a product of extraordinary strength, com-- parable to that of kraft and of a whiteness comparable to that of bleached sulphite pulp. I have discovered, moreover, that the alkaline treatment also imparts a softness to thepulp, and accordingly results in a product which shows a considerably higher tear test than the initial strong sulphite pulp.

In the practice of the present invention, wood chips (spruce, for example) are cooked in a digester, in an acid sulphite solution, preferably sodium bisulphlte (or an equivalent bisulphite solution of zinc,

magnesium, potassium, or other suitable metal) having a relatively hi h combined SO, content, say 3% to 4%, with free SO no greater in proportion than the combined S0,, at a'temperature of about 320 F., and

a pressure of about 75' to ounds. certain cases, however, it may e desirable to commence the digestion of'the chips in an acid liquor containing, say, about 7% free SO and 4% combined S0 and relieve the digester of the excess free SO during the early stages of the cook; Such procedure,

while'substantially equivalent to that. outlined, gives better; penetration of the S0 into th chips and tends to promotea more rapid and efficient liberation of fiber. Due

to such higher combined SOcontent than is ordinarily present in cooklng a sulph1te stock, the hydrogen ion concentration or activity of free S0 is suppressed sufiiciently to avoid the usual rather drastic hydrolysis which occurs in cooking sulphite stock, and hence the average length of fiber is greater than when the acid cooking is carried on with a liquor having a normal or usual con1- bined SO content; whereas, on the other hand, the pulp contains much of the fiber that in an alkaline digestion is dissolved in the alkaline liquor; that is, the strong sulphite pulp contains a greater proportion of the wood than is contained in either sulphate or commercial sulphite pulp. For example, in ordinary commercial operation with the usual acid hisnlphite digestion, the yield of pulp is from 43% to 47% of the weight of the dry wood, and in ordinary sulphate or soda digestion, the yield is from 10% to 47% of the weight of dry wood; whereas,- according to my process, the yield is from to Hence, in addition to cellulose fibers, the strong pulp. as indicated. contains a large proportion of lignin and pontosan, the latter by its presence doubtless imparting thereto its high strength characteristic. and the former its property of diilicult bleachability.

After the completion of the MA" digestion (that is, a digestion in which the acid liquor contains approximately 4% combined and 4% tree S0,) the digestcr contents are blown into a suitable blowpit. The pulp is separated from the spent liquor, and is washed and, if desired, screened as ordinarily, to remove specks, shires, and other like foreign material. The hot spent liquor may be treated, as described in the copending application, for the recovery of heat and chemicals. The washed and preferably screened pulp is then treated at a stock density of about 10%, at approximately room temperature, with an oxidizing of lignin-removing chlorine-containing solution such as one having about 2% to 1% free chlorine, or 3.5% to 7% of 35% bleach (CaOCl based on the bonedry weight of fiber, for about ten to thirty min-' The mild treatment with the onidiz utes. ing solution promotes a reaction between the ligneous and coloring substance contained in the pulp wlth the chlorine. forming soluble The washed and treated stock is thickened to the desired consistency.-as on a wet. ma-

chine, and is mixed with an alkaline solution, preferablyv of a soluble alkali or alkaline earth metal hydroxide, such as caustic soda, containing about 6% to 8% caustic soda, based on the bone-dry weight. of fiber, to a consistency desired for carrying out the alkaline digestion, say about 10%. v The stock is digested in open tanks under atmospheric pressure, at about; 180 'to 212 I i, for about two to seven hours (depending on the characteristics of the particular stock undergoing treatment and the temperature and concentration of sodium hydroxide emp'loyed in digestion) as by blowing live steam thereinto, preferably while being agitated so as to promote a uniform reaction and give a uniform product. \Vhile .I. have found it more economical and expedient to carry out the alkaline digestion in open tanks, it may be stated that equally effective results may be accomplished by digesting the pulp at an elevated temperature in closed digesters, under pressure. The alkaline digest-ion is also a more or less selective reaction. the less rcs'stance cclluloses, the ligneous, resinous, and other coloring matters surviving the chlorine treatment being dissolved by the caustic soda solution and removed from the pulp without materially affecting its strength or pentosan contpnt. The alkaline digestion, moreover imparts a softness to the pulp, doubtless due to the removal of bleach-consuming constituents and less resistant celluloses' therefrom. and the pulp accordingly shows a materially increased tear test. After the alkaline digestion. the stock is washed substantially free from spent liquor and the reaction prod ucts. The spent liquor (which may vary in color from weak tea, in the case of low resinous wood, to a dark brown color. in the case of morehighly resinous woods) may be treated for the recovery of chemicals.

The washed pulp is thenbleached Without injury to its fiber strength, by subjecting it from five to ten hours, at a stock density ofiibout 10%, preferably at room temperature. to a hypochlorite bleach solu' tion containing 12% to 15% of 35% bleach (moon). based; on the bone-dry weight of fiber. The bleached pulp is then washed substantially free from bleaching solution and reaction products. and may be treated with a solution of anti-chlor, as for example. sodium bisulphite, to react with and neutralize the last traces of bleach. The neutral pulp may then be ashed free of reaction products. In certain cases it may be desirable to increase the whiteness of the pulp to a maximum. This may be accomplished by treating the bleached and preferably washed pulp without injury to its fiber strength. at approximately room temperature. at a stock density of 6% to 8%, for about two to six hours, with a chlorine solution containing about 0.3% chlorine,

based on the bone-dry weight of fiber. Such superbleaching treatment increases the color .toabout 100 to 104.

93 Usual bleached alkaline if bleached 4-4 andh g igl. sulppil c pulp li i i t rgnt pulp p p mcnt Mullen strength (beaten). 155-190 162-170 155-100 115 Slowness in min. (beaten). 3- 10. 3-12 5-9 10-15' Alpha cellulose 8110+ 90. 1+ -37 Fl er length (mm); .S O-fin .90-.95 .7-.9 .(i. 7 Teal 140-160 240-250 220-245 ltd-=60 Color Light. 100 Brown. Light. Pentosan 5. 0-7. 5 5-6 7-10 3. 0-41 5 Yield (based on initial WOOd) 50-55 16% 42-45% 454711;

content of my pulp, while slightly lower than kraft, together with a long fiber, doubtless impart-s thereto its extraordinary strengtln It is to be noted that the yield is equal to that obtained by the usual sulphite process, and that the treatment of the 44 pulp while-materially increasing its alpha cellulose content, tear, and color, does not materially affect its strength or pentosancontent. In other words, the final pulp possexes the most valuable properties incident to both kraft and sulphite pulp, and may hence be beaten, sized and manufactured into high grade bond and ledger papers. The paper does not change color upon reasonable ageing in the air or upon exposure to light, and is substantially similar to ordinary sulphite paper in these respects.

I do not herein claim the subject matter of application, Serial No. 716,154, filed May.

27, 1924, or that of application, Serial No. 72,522, filed December 1, 1925 (both filed by Milton 0. Schur and myself), which disclose the liberation of the usual sulphite pulp with the usual sulphite liquors, folowed by a refining orpurifying rocess to produce a high alpha cellulose fi er which comprises a pretreatment of the pulp with an oxidant, followed by digestion in an alkaline solution and bleaching. The sulphite liquors herein employed, in which the free S0 is not in excess of combined S0 and in which the combined SO amounts to about 3% to 4%, produce sulphite pulp which when treated by the process of the treatln present invention yields a product having different characteristics than those products produced from pulp liberated with the usual sulphite liquors. Y

What I claim is:

1. A process which comprises digesting raw cellulosie material in an acid sulphite liquor in which the free so, is notin excess of combined 80 washing the resulting pulp substantially free from its digesting liquor, said washed pulp with a solution containing chlorine, washing said chlorinetreated pulp, and digesting said washed and I treated pulp at an eleyated temperature in an alkaline solution.

2. In the manufacture of a pulp of high strength and whiteness, a process which comprises treating sulphite pulp, formed by previous digestion of raw cellulosie material in an acid sulphite liquor containing free and combined SO in such proportion that such sulphite pulp is of extraordinary strength, with an oxidizing solution; washing said treated pulp: digesting said washed and treated pulp at an elevated temperature in an alkaline solution; washing said digested pulp: and bleaching said washed pulp, said process substantially unafi'ecting the strength of the sulphite pulp.

3. In the manufacture of a pulp of high strength and whiteness, a process which comprises treating sulphite pulp; formed by previous digestion of raw cellulosic material in an acid sulphite liquor containing free and combined SO in such proportion that such sulphite pulp is of extraordinary strength, with an oxidizing solution; washing said treated pulp; digesting said washed and treated pulp at an elevated temperature inv an alkaline solution; washing said digested pulp, bleaching said washed pulp, and superbleachin said bleached pulp, said process substantlally unatfecting the strength of the sulphite pulp. v

4. In the manufacture of a pulp of high strength and whiteness, a process which comprises treating sulphite pulp, .formed by previous digestion of raw cellulosic material in an acid sulphite liquor containing free and combined SO. ,in such proportion that such sulphite pulp is of extraordinary strength, with a solution containing chlorine; washing said treated pulp, digesting said washed and treated pulp in asolution of sodium'hydroxide; washing said digested pulp; and bleaching said washed pulp in hypochloritebleach liquor, said process substantiallylunaftecting the strength 'of the sulphite pulp.

5. In the manufacture of a pulp. of high strength and whiteness, a process which comprises treating sulphite pulp, formed by previous digestion ofraw cellulosic material in an acid sulphite liquor containing free and combined SO in such proportion that such sulphite pulp is of extraordinary strength, with a solution containing chlorine; Washing said treated pulp, digesting said washed and treated pulp in a solution of sodium hydroxide; washing said digested pulp; bleaching said Washer] pulp in hypochlorite bleach liquor, and superbleaching said bleached pulp in a chlorine solution, said process substantially unaltecting the strength of the sulpliite pulp.

6. In the manufacture of a pulp oi high strength and whiteness, a process which comprises treating sulphite pulp, formed by previous digestion of raw cellulosic material in an acid sulphitc solution in which the free and combined SO are in approximately equal proportions of 3% to 45% each with a solution containing about 2% to 4% chlorine based on the dry weight of fiber at. about room temperature for about ten to thirty minutes; washing said treated pulp; digesting said washed and treated pulp in a caustic soda solution containing 6% to 8% caustic soda based on the dry weight of fiber at abo t 180 to 212 F. for about two to seven hours. washing said digested pulp; and bleaching said pulp in a hypochlorite bleach solution containing 12% to 15% of bleach (CaOC-h) based on the dry weight of fiber, at approx mately room temperature for about five to ten hours.

7. In the manufacture of a pulp of high strength and whiteness, a process which comprises treating sulphite pulp, formed by previous digestion of raw cellulosic material in an acid sulphite solution in which the free S0 and combined SO are in approximately equal proportions of 3% to 4% each with a solution containing about 2% to 4% chlorine based on the dry weight of fiber at about room temperature for about 1 merges? I ten to thirty minutes; washing said treated pulp; digesting said Washed and treated pulp in a caustic soda solution containing 6% to 8% caustic soda based on the dry weightof fiber at about 180 to 212 F. for about two to seven hours, washing said digested pulp; bleaching said pulp in a hypochlorite bleach solution containing 12% to 15% of 35% bleach (CaOCh) based on the dry weight of fiber, at approximately room temperature for about five to ten hours, and superbleaching the bleached pulp in a chlorine solution containing about 0.3% chlorine based on the dry weight of liber approximately at room temperature from two to six hours.

A process which comprises digesting raw cellulosic material in an acid sulphite in which the tree S0 and combined are in approximately equal proportions of 3% to 1% each under conditions of high tern-- perature and pressure, separating and washing the resultin pulp substantially free from its digesting liquor, treating said washed pulp with a solution containing chlorine, washing said chlorine-treated pulp,

and digesting said washed and treated pulp at an elevated temperature in a solution of sodium hydroxide.

9. ln the manufacture of a pulp of high strength and whiteness, a process which comprises treating sulphite pulp, formed by previous digestion of raw 'cellulosic material in an acid sulphite solution containing about 3% to 4% combined S0 with a solution containing chlorine, washin said :treated pulp, and digestingsaid was ed and treated pulp at an elevated temperature in an alkaline solution.

In testimony whereof I have affixed my signature.

GEORGE A. RICHTER. 

